Applicants"" invention relates generally to radiocommunication systems, e.g., cellular or satellite systems and, more particularly, to techniques for supporting and enhancing emergency calling procedures in such systems.
The growth of commercial radiocommunications and, in particular, the explosive growth of cellular radiotelephone systems have changed the ways in which people communicate. One survey indicates that about 80% of the people who purchase mobile communication units and service subscriptions do so to enhance their personal security. Presumably, many of these subscribers would expect to use their mobile units to aid them in urgent situations, e.g., when their vehicle has become disabled or in an emergency situation requiring rapid medical and/or police response. In these circumstances it would be desirable that the radiocommunication system be able to independently determine a location of the mobile unit, particularly in the case where the subscriber does not know his or her precise location. Moreover, it is expected that the FCC will soon require that network operators forward the position of an emergency caller to the emergency service provider.
There are many techniques available to generate mobile unit location information. In a first category, the mobile unit could estimate its own position and send a message with its coordinates when placing an emergency call. This could be accomplished by, for example, providing the mobile unit with a Global Positioning System (GPS) receiver that receives location information from the GPS satellite network. The mobile unit can then transmit this information to the system, which would then forward it to the emergency service provider. This requires, however, significant modification of existing mobile units to include GPS receivers, as well as additional signalling between the mobile units and base stations.
Alternatively, the base stations which transmit signals to, and receive signals from, the mobile units could be used to determine the mobile unit""s location. Various techniques, including attenuation of a mobile unit""s signal, angle-of-arrival, and difference between the time-of-arrival (TDOA) of a mobile unit""s signal at different base stations, have been suggested for usage in providing mobile unit location information. See, for example, the article entitled xe2x80x9cTime Difference of Arrival Technology for Locating Narrowband Cellular Signalsxe2x80x9d by Louis A. Stilp, SPIE Vol. 2602, pp. 134-144. These solutions also have their drawbacks including the need to modify the many existing base stations, e.g., to provide array antennas to support angle-of-arrival techniques or to synchronize base station transmissions to support TDOA techniques.
A third category of strategies for locating mobile units in radiocommunication systems involves the provision of an adjunct system, i.e., a system which may be completely independent of the radiocommunication system or which may share various components (e.g., an antenna) with the radiocommunication system but which processes signals separately therefrom. This may be advantageous, for example, as an expedient solution to providing mobile unit location without modifying the large number of existing base stations in a system. For example, consider the equipment illustrated in FIG. 1 wherein the adjunct scanning units are not co-located with the base stations of radiocommunication system. Therein, a base station 1 supports radiocommunication within cell 2 and, in particular with mobile unit 3. An adjunct system, partially shown by way of scanning units 4, 5 and 6, monitors accesses to the system by mobile unit 3. When mobile unit 3 makes an emergency access, adjunct units 4, 5 and 6 can detect this emergency access by, for example, the presence of a set emergency flag in an origination message or based upon the dialed number. The adjunct units can then use the mobile unit""s transmissions on either a control channel or a traffic channel to provide information to a location processing center 7. The location processing center then uses the information provided by the various adjunct units to, for example, triangulate the position of mobile unit 3 and report this position to an emergency service center 8. More details regarding exemplary usages of adjunct systems can be found in U.S. Patent No. 5,327,144 to Stilp et al., entitled xe2x80x9cCellular Telephone Location Systemxe2x80x9d, the disclosure of which is incorporated here by reference.
However, one problem associated with the use of adjunct systems involves the desirability of the emergency center mapping the location of a mobile unit provided by the adjunct system with the emergency call received from the radiocommunication system. Consider this issue in the context of systems operating in accordance with IS-136, which has three types of mobile identifiers. The mobile identification number (MIN) is the same identifier as used in the older analog AMPS system and is a binary representation of the mobile station""s phone number, which is also referred to as the directory Number (DN). Given the need for global mobility, however, the differences between these and other existing techniques for identifying mobile units need to be reconciled.
This, among other reasons, lead to the development of the International Mobile Station Identifier (IMSI), which is a decimal identifier number having a maximum of 15 decimal digits. Although IMSI is not a dialable number, it does solve the problem of international roaming and is transmitted over the air interface as a mobile station identification (MSID) encoded to 50 bits. Due to the length of IMSI, however, a single page message transmitted by the radiocommunication system can carry three pages to mobiles using MIN but only one page using IMSI. Thus, using IMSI reduces the paging capacity (pages per time unit) per control channel.
In order to overcome this problem and further increase the paging capacity compared with the MIN, but still provide the capacity for international roaming, the Temporary Mobile Station Identifier (TMSI) was introduced. In IS-136 the length of TMSI is 20 or 24 bits which results in four or five pages per paging message, respectively. The TMSI is assigned to a mobile station in a TMSI assignment procedure after which the mobile station can be reached by the cellular system using the TMSI. When the mobile unit travels to a xe2x80x9cnewxe2x80x9d system, the mobile unit is reassigned a new TMSI controlled by the xe2x80x9cnewxe2x80x9d system. In addition to being used to contact a mobile unit, the same identity type is typically used when the mobile station contacts (i.e., makes an access attempt with) the cellular system. Thus, for mobile units having an assigned (also referred to as xe2x80x9cactivexe2x80x9d) TMSI, this may conventionally be the identifier which is used in access messaging between the mobile unit and the system.
The radiocommunication system has the capability to translate these identifiers into the mobile unit""s dialable number and provide both the emergency call and the dialable number to the emergency service center. However, usage of IMSI and TMSI poses a problem for an adjunct positioning system. Specifically, the xe2x80x9cphone numberxe2x80x9d of the mobile unit 3 is typically not present in the signalling which is monitored by the adjunct scanning units 4, 5 and 6 and the adjunct system cannot keep track of the various TMSIs assigned by the radiocommunication system. Thus, although the adjunct system can estimate the position of the mobile unit, e.g., as described in U.S. Pat. No. 5,327,144 to Stilp et al., the adjunct system can not forward the xe2x80x9cphone numberxe2x80x9d to the application, e.g., emergency service center 8. Thus, emergency service center 8 will have difficulty mapping the position information received from the adjunct system with the emergency call received from the radiocommunication system. Accordingly, it would be desirable to provide techniques for the emergency service center to receive information regarding the phone number associated with a mobile unit making an emergency access to a radiocommunication system.
According to exemplary embodiments of the present invention, these drawbacks and limitations of handling emergency calls are overcome. For example, upon recognizing that an emergency call is to be made, a mobile unit can ignore typical access rules and use its MIN as part of its system access messaging. In this way, the adjunct system can acquire and forward this information to the appropriate emergency service center. Alternatively, or in addition thereto, the system can use the MIN to address messaging in the downlink, which could also be monitored by the adjunct system.
According to another exemplary embodiment, the mobile unit may, upon recognition of an emergency call, use its predefined mobile station identity type for addressing during system access, but include its dialable number as part of the access payload. Similarly, the system (i.e., the base station) can transmit that mobile unit""s dialable number as an address of a response to an emergency access attempt. In either case, the adjunct system can read this information and forward same to the emergency service center.